This application is based on Patent Application No. 11-129548 filed in Japan, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a telecentric optical system, and further relates, for example, to a three-dimensional measuring device using a telecentric optical system as a projection optical system (e.g. an object lens used for shape/dimension measuring) when a constant magnification is required even when the working distance (distance from an object to the outermost lens on the object side) and back focal length (distance from the outermost lens surface on the image side to the image plane) change.
2. Description of the Related Art
The confocal detection method is known as one type of method used for three-dimensional shape measurement. The pin-hole method and the slit method are among the confocal detection methods. The pin-hole method uses an illumination pin-hole for emitting light to illuminate an object, and a detection pin-hole for directing the light reflected from the illuminated object to a sensor. The illumination light emitted from the illumination pin-hole passes through a beam splitter and optical system and illuminates the object. Then after the light reflected from the object passes through the optical system, the light is reflected by the beam splitter and enters the detection pin-hole. The illumination pin-hole and the detection pin-hole are arranged at conjugate positions (i.e., optically equivalent positions in an objective optical system), such that when these pin-holes are together at conjugate positions (focal position) relative to both objects, a maximum amount of light passes through the detection pin-hole. If the working distance from this state changes, the amount of light passing through the detection pin-hole is markedly reduced. Accordingly, the object dimension (height) can be detected with great accuracy in a direction parallel to the optical axis.
Since height information can only be obtained for one point at a time in the pin-hole method, when actually measuring the three-dimensional shape of an object, the state of the conjugate position relationship of the pin-hole and object must be detected at each point, and a two-dimensional scan within a plane perpendicular to the optical axis of the optical system or object must be performed while obtaining each height information. The slit method eliminates this disadvantage. Since the scan is unidimensional in the slit method, the object height measurement accuracy can be sufficient while reducing the scan mechanism since height information is obtained at many points at once using this method. However, in realizing the slit method, the system normally must be telecentric on both the object side and image side within a range providing adequate image height for projection of the length of the slit, i.e., the optical system must satisfy the required specifications.
Since the object dimension is detected in a direction parallel to the optical axis in the confocal detection method, generally, the scan method used is either a scan method which moves the measured object (i.e., the subject of the measurement) in the optical axis direction, or a scan method which moves the optical system in the optical axis direction. In the case of the former scan method, the measurement object is restricted due to the increase in the load on the scanning system due to the size and weight of the measurement object. Moreover, the measurement object must be completely stationary to achieve precise measurement.
In the case of the latter scan method, the object image normally must be projected on the image plane at the same magnification even if the conjugate distance (i.e., distance from the object to the image) changes at the image height position. Therefore, the optical system normally must have telecentric characteristics on both the object side and image side even when the conjugate distance changes. When using a method which detects the focus state by moving the entire dual-conjugate optical system in the optical axis direction, the moving members become larger, and the load increases on the scanning mechanism. Accordingly, in devices constructed as described above, the moving lens unit must be rendered as small and light weight as possible.
U.S. Pat. No. 4,592,625 discloses an optical system that normally has telecentric characteristics on both the object side and image side even when the conjugate distance changes. This optical system comprises an afocal system of a plurality of lens units, and two positive lens units arranged on both sides thereof on the same axis. One positive lens unit and the afocal system are constructed so as to be independently movable, such that when the conjugate distance changes the working distance and the back focal length are maintained normally constant by moving the positive lens unit and the afocal system in a predetermined relationship thereby maintaining the conjugate relationship of the two units.
In the optical system disclosed in U.S. Pat. No. 4,592,625, the two lens units of one positive lens unit and the afocal system must be moved. Two methods are considered to move the two lens units: a method of independently moving each lens unit using two drive systems, and a method for moving the two lens units using a single drive system linking the two lens units via a cam or the like. Using either method, however, makes the device complex, and the optical system does not meet its purpose in repeatedly changing the conjugate distance at high speed with high accuracy. Since this optical system is constructed so as to change the overall length of the optical system while normally maintaining a constant working distance and back focal length, it cannot be used in systems to change the working distance and back focal length.
An object of the present invention is to provide an improved telecentric optical system.
A further object of the present invention is to provide a telecentric optical system capable of focusing while maintaining telecentric characteristics on both the object side and the image side by moving one lens unit even when the conjugate distance changes.
A still further object of the present invention is to provide a three-dimensional shape measuring device capable of excellent measurement even when the working distance and back focal length change.
These objects are attained by a telecentric optical system comprising sequentially from one conjugate side:
a first lens unit in the form of an afocal optical system including sequentially from one conjugate side a first subunit having positive optical power and a second subunit having positive optical power, and having a stop fixed in the optical axis direction at the focal position on the back side of the first subunit, and a second lens unit in the form of an afocal optical system that is movable in the optical axis direction to adjust the focus while maintaining a telecentric state.